Handbook for Irrigation of Wine Grapes in South Africa

As the soil dries out, and water uptake becomes slower, the daytime water potential gradient increases (Fig. 6.7). Since the daytime water deficit in the grapevines increases, more water needs to flow from the cells into the transpiration stream. Consequently, shrinking of the plant cells also increases as the plant available water becomes less (Fig. 6.8). Details of measuring the diurnal shrinkage are presented in Chapter 8. If the soil becomes so dry that the roots cannot absorb enough water during the night to return the cells to their turgid state, grapevines will show signs of wilting the next day. If the wilting symptoms do not recover upon irrigation, the soil water content at this particular stage is referred to as permanent wilting point.

Transpiration

A

B

Transpiration

Water uptake

Water uptake

FIGURE 6.7. Schematic illustration where the vertical arrows indicate the water potential gradient in grapevines in (A) wet soil and (B) dry soil.

150 CHAPTER 6 – GRAPEVINE WATER STATUS